In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these higher device densities there have been, and continue to be, efforts toward scaling down the device dimensions on semiconductor wafers. This continuing trend has also led to advanced monitoring and quality control of every step of the semiconductor manufacturing process.
High resolution lithographic processes are used to achieve small features. In general, lithography refers to processes for pattern transfer between various media. In lithography for integrated circuit fabrication, a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist. The film is selectively exposed with radiation (such as optical light, x-rays, or an electron beam) through an intervening master template, the mask, forming a particular pattern. Exposed areas of the coating become either more or less soluble than the unexposed areas (depending on the type of coating) in a particular solvent developer. The more soluble areas are removed with the developer in a developing step. The less soluble areas remain on the silicon wafer forming a patterned coating. The pattern corresponds to the image of the mask or its negative. The patterned resist is used in further processing of the silicon wafer.
Within lithography, patterns are transferred from a photomask or reticle onto a photoresist layer which overlies the film on the wafer through an exposure process. If the photomask or reticle contains defects, even submicron in range, such defects may be transferred to a wafer during the exposure. Such defects may be generated by the fabrication process utilized to produce the mask or reticle as well as during subsequent handling and processing. Such defects generally fall into two classes: fatal (or killer) defects and nonfatal defects.
Critical dimensions of the patterned resist, such as line widths, affect the performance of the finished product and are sensitive to processing conditions. Processing conditions that affect critical dimensions include conditions relating to resist application, pre-baking, resist exposure, post-baking, and resist development. A few degrees variation in the pre-bake temperature, for example, can have a significant affect on critical dimensions. Many of the conditions that affect critical dimensions are difficult to control, often resulting in variations from batch to batch.
Techniques, equipment and monitoring systems have concentrated on preventing and reducing defects within the lithography process. For example, aspects of the resist process which are typically monitored are: whether the correct mask has been used; whether resist film qualities are acceptable (e.g., resist is free from contamination, scratches, bubbles, striations, etc.); whether image quality is adequate (e.g., good edge definition, linewidth uniformity, or indications of bridging); whether critical dimensions are within the specified tolerances; whether defect types and densities are recorded; and whether registration is within specified limits.
Within the lithography process, two automated areas of defect detection have been concentrated upon: electrical signal analysis and image analysis. By using an electrical signal analysis, defects such as “opens” in circuitry, unwanted electrical bridges, and electrical failures can be detected within the silicon wafers. Image analysis can consist of overlay inspection (OL) and critical dimension inspection (CD), which are used to determine the quality of the lithography process. The OL inspection measures the registration of consecutive layers of multi-layer semiconductor chips. During the inspection, the wafer is moved to an optical microscope. Under this optical microscope the position of marks or targets of the previous processed layer are measured against the marks of the layer that is currently being added. The CD inspection measures the layer line-widths. The wafer is moved to a high-resolution CD-SEM (Critical Dimension Scanning Electron Microscope) where the line-width is measured and determined to be within a threshold or pre-determined tolerance.